Any music lover or sound engineer can appreciate the importance of having a good audio amplifier in any high-fidelity stereo system. A good amplifier typically has good physical characteristics, i.e., good frequency response, low distortion, etc. and/or delivers good sound quality. Having good physical characteristics, however, does not necessarily lead to producing sound of good quality. While sound quality is not as easily measurable or quantifiable like the physical characteristics, it is a well-known fact that an amplifier with good physical characteristics still may not yield good sound quality due apparently to errors or distortions which are not easily observable from studying the amplifier's physical characteristics. The reasons for this phenomenon and the solutions for it, however, are not fully understood at this point. See, e.g., U.S. Pat. No. 4,809,336 and U.S. Pat. No. 5,467,400.
There are different types of high-fidelity amplifiers in the market. But most notably, the types can be broadly differentiated as having either vacuum tubes or solid state devices such as transistors. For the transistor-based amplifiers, many of them employ an overall negative-feedback (NFB) topology which is a well known scheme to those skilled in the art. Each of these types has advantages and disadvantages. For instance, it is generally known that the transistor-based amplifiers employing NFB topology have good physical characteristics when compared to the vacuum tube transistors. Some expensive vacuum tube amplifiers have much worse total harmonic distortion (THD) and narrower frequency range, for example. On the other hand, many people believe that the vacuum tube amplifiers still deliver better sound quality. Similarly, some of the well-designed transistor-based amplifiers which do not employ the NFB topology have poorer physical characteristics, but are still touted as producing higher quality sounds.
While it would be easy to point to the NFB topology as the cause of the poorer sound quality, the exact cause and nature of sound degradation are still not fully understood. And although the understanding in regard to the phenomenon of sound quality versus physical characteristics is incomplete, it is clear that the ideal is to have an amplifier which has good physical characteristics and still delivers good sound quality. However, the industry has had difficulty developing an amplifier which meets both criteria.